Strain gauge with resonant light modulator

Abstract

A strain gauge for measuring strain in a structural member, including: a light modulator adapted to be attached to the structural member, further including: a plurality of deformable elements, each of said deformable elements having a reflective surface and a resonant frequency that varies as a function of strain on the element; means for exerting a force to the deformable elements to cause them to deform at their resonant frequency between first and second operating states, an optical system for directing incident light onto the light modulator, and directing modulated light from the light modulator to a sensor that provides an output signal that varies as a function of the resonate frequency of said deformable elements; and means for generating a representation of the strain in the structural member from said output signal.

What is claimed is:  
     
       1. A method for measuring strain in a structural member comprising: 
       (a) mounting a light modulator comprising deformable elements having reflective surfaces and a resonant frequency that varies as a function of strain on the deformable elements on the structural member;  
       (b) applying a force to the deformable elements to cause them to deform at their resonant frequency between first and second operating states;  
       (c) directing incident light onto the light modulator and directing modulated light from the modulator to a sensor that produces an output signal that varies as a function of the frequency of the modulated light; and  
       (d) generating a representation of the strain in the structural member from the output signal, wherein the strain in the structural member is determined in accordance with the following relationship:            Δ                 L     L     ≈     2        σ   E          (         f   res       f   res   0       -   1     )                       
       wherein σ and E are a residual tensile stress and Young's modulus of the deformable elements, f res  and f 0   res  are the resonant frequencies of the deformable elements with and without strain, respectively, and ΔL/L is the strain of the deformable elements.  
     
     
       2. A method in accordance with  claim 1 , further comprising extracting a frequency dependent signal from the output signal produced by the sensor and generating the representation of the strain in the structural member from the frequency dependent signal.